Laser light energy and dose control using repetition rate based gain estimators

ABSTRACT

A laser system&#39;s laser light energy control and resulting dose control is improved by creating and using a set of gain estimators, one for each of a set or range of laser light pulse repetition rates. When a new repetition rate is used, its corresponding gain estimator is retrieved, used to compute the voltage to fire the laser source, and updated. The resulting generated laser light thereby avoids the convergence delay inherent in prior laser systems and, further, can repeatedly do so with subsequent specified repetition rates.

BACKGROUND Field of the Invention

The disclosed subject matter is in the field of laser light control andmore specifically in the field of controlling the energy of a lasergenerated by a laser light source as may be used in semiconductorphotolithography processes.

Related Art

Photolithography is a commonly used process in the semiconductorindustry. Modern photolithography typically uses a laser light source,also known as a laser system, to provide very narrow band light pulsesthat illuminate a mask thus exposing photo-resistive material on siliconwafers, also known as substrates, in stepper-scanner devices, also knownas scanners. Advances in semiconductor device parameters have putincreasing demands on the performance characteristics of the laser lightsources and stepper-scanners used. Improvements in precision and speedof operation of these devices are increasingly needed.

As is known in the art, the stepper-scanner device periodicallycommunicates desired laser light parameters to the laser light source toachieve a desired dosage of laser light energy for use in thephotolithographic process. In turn, the laser light source generates thelaser light and outputs it to the stepper-scanner. There are, however,numerous challenges in performing these operations. For example, it cantake some time for the generated laser light to stabilize on the desiredparameters. Further, because of noise and other disturbances in thelaser light source, it can be difficult to accurately generate the laserlight at the desired energy level. What is needed are furtherimprovements in how the laser light source quickly and accuratelygenerates the laser light to meet the desired parameters.

SUMMARY

A system and method for laser light energy control and resulting dosecontrol creates and uses a set of gain estimators, one for each of arange of laser light pulse repetition rates. When a new repetition rateis specified, its corresponding gain estimator is retrieved and used toadjust and fire the laser source. The resulting generated laser lightthereby reduces convergence delay and, further, can repeatedly do sowith subsequently specified repetition rates.

In one embodiment is a method of laser light energy control comprising:receiving, in a laser system controller, a first laser trigger commandand a voltage command; converting, by the laser system controller, thevoltage command to a first energy target; determining, by the lasersystem controller, a first laser repetition rate based on a differencebetween the first laser trigger command and a previous laser triggercommand; retrieving, by the laser system controller, a first repetitionrate gain estimator corresponding to the first laser repetition rate;determining, by the laser system controller, a first laser voltage usingthe first energy target and the first repetition rate gain estimator;directing, by the laser system controller, a laser source to fire usingthe first laser voltage; receiving, in the laser system controller, asubsequent laser trigger command and a subsequent voltage command;converting, by the laser system controller, the subsequent voltagecommand to a second energy target; determining, by the laser systemcontroller, a second laser repetition rate based on a difference betweenthe subsequent laser trigger command and the first laser triggercommand, wherein the second laser repetition rate is different than thefirst laser repetition rate; retrieving, by the laser system controller,a second repetition rate gain estimator corresponding to the secondlaser repetition rate; determining, by the laser system controller, asecond laser voltage using the second energy target and the secondrepetition rate gain estimator; and, directing, by the laser systemcontroller, the laser source to fire using the second laser voltage.

In another embodiment is a laser system for laser light energy controlcomprising: a laser system controller configured to: receive a firstlaser trigger command and a voltage command; convert the voltage commandto a first energy target; determine a first laser repetition rate basedon a difference between the first laser trigger command and a previouslaser trigger command; retrieve a first repetition rate gain estimatorcorresponding to the first laser repetition rate; determine a firstlaser voltage using the first energy target and the first repetitionrate gain estimator; direct a laser source to fire using the first laservoltage; receive a subsequent laser trigger command and a subsequentvoltage command; convert the subsequent voltage command to a secondenergy target; determine a second laser repetition rate based on adifference between the subsequent laser trigger command and the firstlaser trigger command, wherein the second laser repetition rate isdifferent than the first laser repetition rate; retrieve a secondrepetition rate gain estimator corresponding to the second laserrepetition rate; determine a second laser voltage using the secondenergy target and the second repetition rate gain estimator; and, directthe laser source to fire using the second laser voltage.

BRIEF DESCRIPTION OF THE DRAWING

Various embodiments are disclosed in the following detailed descriptionand the accompanying drawing.

FIG. 1 is a block diagram of an exemplary laser system and scanner asmay be used in one embodiment.

FIG. 2 is a graph showing characteristics of a generated laser light asmay be used in one embodiment.

FIG. 3 is a block diagram of a laser system controller as may be used inone embodiment.

FIG. 4 is a flowchart of various embodiments of the present method ofsetting, updating, and using a set of gain estimators.

DETAILED DESCRIPTION

Described herein is a system and method for controlling the energy of alaser light source generated laser. In the present approach, the desiredenergy of the laser light is achieved more rapidly and more accuratelythan with known approaches thus improving dose control in thephotolithography process. The operations and elements of this system andmethod will now be described.

Referring now to FIG. 1, a block diagram 100 of a laser system 110 andscanner 140 as may be used in a modern deep ultraviolet (“DUV”, e.g.,having wavelengths in the range of 5-250 nanometers (nm)photolithography process and with the present approach can be seen. Thesource of the laser light in laser system 110 is a laser source 120,which can be a single or dual chamber system, controlled by a controller130 via communication 125. When laser source 120 fires the resultinglaser light 115 travels to scanner 140, which is responsible forexposing the semiconductor wafer.

As is known in the art, scanner 140 sends to laser system 110 viacommunication 135 desired parameters for the laser light to be generatedby laser system 110. The parameters, intended to achieve a desiredexposure in the photolithography process, typically include such thingsas laser light wavelength, energy level and a timing trigger for whenthe laser system is to fire the laser. In turn, the laser systemgenerates the laser light 115 based on those parameters. This processthen continues as scanner 140 communicates additional desired parametersfor further laser light to be generated by laser system 110.

As is known in the art, there is a relationship between the amount ofvoltage applied to laser source 120, as specified by controller 130 viacommunication 125, and the resulting energy in the generated laserlight. This relationship is commonly represented as dv/de, which is thederivative of voltage with respect to energy. U.S. Pat. No. 7,756,171and U.S. patent application Ser. No. 11/900,527, each incorporatedherein by reference, describe these aspects known in the art. It isknown in the art to use what is called a gain estimator, an algorithmfor estimating the dv/de relationship. As stated above, there is arelationship between the amount of voltage applied to laser source 120and the resulting energy in the generated laser light. In general, thegreater the applied voltage the greater the resulting energy. However,this relationship is affected by noise and other disturbances in thelaser system. As such, rather than simply presuming a given voltage willalways result in the same energy, a gain estimator algorithm is createdby varying (or dithering, i.e., dv) the voltage locally and then usingthe corresponding response in energy (i.e., de) to compute de/dv as anoutput. This de/dv, which is the output of the gain estimator, is thenused to compute the final voltage value that the laser system then usesto generate the laser light at the desired energy level. U.S. Pat. No.8,102,889, incorporated herein by reference herein, describes theseaspects known in the art.

However, while such known usage of a gain estimator can reduce oreliminate error or variability of the laser light energy, it still failsto address the problem of the generated laser light not providing thedesired energy level during the transient phase or convergence lag. Inother words, should the scanner request laser light at a differentrepetition rate (i.e., the number of laser pulses fired in a second,hereinafter “RepRate” or “rr”, as dictated by the timing triggerreceived from scanner 140), laser energy performance will be temporarilyworse until the gain estimator finds the new de/dv again because, asshown in FIG. 2, RepRate affects the de/dv gain.

Referring now to FIG. 2, characteristics of the generated laser lightcan be seen in a graph form. In this example, scanner 140 hascommunicated a series of differing timing triggers which resulted inlaser system 110 generating laser light at a RepRate of 2000, then 6000,then 5990 and then 4000 laser pulses per second. As shown, the actualde/dv (the inverse of the above-described dv/de), in degrees representedby a series of x's in the figure, is different at each of the differingRepRates. As also shown, the resulting estimated de/dv, in degreesrepresented by a series of o's in the figure, converges to within asmall deviation from the actual de/dv within approximately 100,000 (100k) laser pulses. Other experiments have shown that it can take up toabout 30 seconds for this convergence to occur (with time=number oflaser pulses divided by laser pulse RepRate, it takes about 16.67seconds for 100 k laser pulses to occur at a RepRate of 6,000 and about50 seconds for 100 k laser pulses to occur at a RepRate of 2,000, thusconfirming that 30 seconds is a reasonable estimate). This convergencelag, that is, the number of laser pulses and/or the time between whenthe laser source is directed to fire at a new RepRate and the generatedlaser light achieves the desired energy level, detrimentally affects thedesired wafer dose in scanner 140.

The present approach overcomes this problem by creating and storing aset of gain estimators each operable for a differing RepRate. Then,whenever the laser system is asked to fire the laser with a differingtiming trigger interval, i.e., at a differing RepRate, a gain estimatorto be operable at that RepRate can be used without waiting forconvergence to the new de/dv. The benefit of this approach is thenfurthered when there are frequent changes in the RepRate because theconvergence lag is avoided with each change.

Referring now to FIG. 3, controller 130 of FIG. 1 is shown having astored RepRate Gain Estimator (“RRGE”) table 301. As shown in table 301,each row is a range of RepRates (e.g., 450-549, 550-649, etc.) with acorresponding RepRate Gain Estimator (e.g., RRGE 1, RRGE 2, RRGE 3,etc.) to be used with a RepRate in that range.

As further explanation, and as would be understood by one of skill inthe art in light of the teachings herein, a gain estimator is acollection of data and a series of steps (i.e., logic and operations)that act on that data and the inputs to output the result. One exampleof such a gain estimator is:

${\begin{bmatrix}{x_{1}\left( {k + 1} \right)} \\{x_{2}\left( {k + 1} \right)}\end{bmatrix} = {{\begin{bmatrix}a_{11} & a_{12} \\a_{21} & a_{22}\end{bmatrix}\begin{bmatrix}{x_{1}(k)} \\{x_{2}(k)}\end{bmatrix}} + {\begin{bmatrix}b_{11} & b_{12} \\b_{21} & b_{22}\end{bmatrix}\begin{bmatrix}{V(k)} \\{E(k)}\end{bmatrix}}}},{{Dedv} = {{\begin{bmatrix}c_{1} & c_{2}\end{bmatrix}\begin{bmatrix}{x_{1}(k)} \\{x_{2}(k)}\end{bmatrix}}.}}$

where each stored gain estimator (i.e., each RRGE in the table) thusincludes this data denoted as values X₁ and X₂), which are the state ofthe estimator that get updated each time the estimator executes, krefers to “value before the update” while k+1 refers to “value after theupdate”, and a, b, and c are constants determined by the system creator.

In this way, any time a new RepRate is desired or specified, again viathe timing trigger received by the laser system from the scanner, acorresponding RRGE can immediately be retrieved and used withoutincurring any convergence lag. It is to be understood that thecorrespondence between RepRate ranges and corresponding RepRate GainEstimators can be stored in any data storage format, structure or layoutand is not limited to the specific form of the table shown in thefigure. Further, the particular RepRate range or quantum of valuescorresponding to a given RepRate Gain Estimator can be as broad ornarrow as desired as they are only limited by available storagecapacity.

Referring now to FIG. 4, a flowchart of a process 400 of setting,updating, and using a set of gain estimators according to variousembodiments will now be described.

In step 401, the gain estimators are set to default values. In oneembodiment, the gain estimators (i.e., the RepRate Gain Estimators) inthe RepRate Gain Estimator Table 301 of laser controller 130 are set todefault values by initially setting the data “X” state values, asdiscussed above with respect to FIG. 3. In one still further embodiment,the default values are set so that the RepRate Gain Estimators willoutput an average of typical dv/de values, which typically range from0.02 to 0.2, and therefore the default values are set so the RRGEs willoutput an average value of 0.11. It is to be understood that otherdefault values can likewise be used and, further, that each gainestimator need not be set to the same default value.

In step 403 a laser trigger (also known as a laser firing trigger) and avoltage command are received from the scanner by the laser system. Inone embodiment this laser trigger and voltage command are received fromscanner 140 by laser system 110 via communication 135 and the voltagecommand is determined by scanner 140 based on a desired laser energylevel and its knowledge of the dv/de relationship, as described above.It is to be understood that the laser trigger and the voltage commandcan be sent as a single communication from the scanner to the lasersource or can be sent as separate communications from the scanner to thelaser source in any sequence or order. In various embodiments, othercommand signals (e.g., digitally encoded signals) may be used in placeof a voltage command.

In step 405, the laser system converts the voltage command to an energytarget. In one embodiment, this conversion is performed by laser system110, and more particularly by laser system controller 130, based on itsgreater knowledge of the dv/de relationship within operations of thelaser system and as is understood by one of skill in the art.

In step 407, the laser system determines the RepRate of the laser. Inone embodiment, this determination is performed by laser system 110, andmore particularly by laser system controller 130, based on a differencebetween the received laser trigger command received from scanner 140 anda previously received laser trigger command from scanner 140.

In step 409, the gain estimator for the determined RepRate is retrieved.In one embodiment, this is performed by retrieving a RRGE n in RRGETable 301 corresponding to the determined RepRate.

In step 411, the voltage with which to fire the laser, referred toherein as the laser voltage, is determined using the energy target andthe retrieved gain estimator. In one embodiment, this is performed bylaser system controller 130 in laser system 110 and, according tovarious embodiments, is performed according to the following formula:V=V _(o) +dV/dE[(E−E _(o))+ϵ(E−E _(o))²]where,

-   V Voltage guessed by the static control-   E Energy Target-   Vo Reference Voltage-   Eo Reference Energy-   ϵ Adjustment due to energy curve-   dVldE First order derivative/slope of voltage vs. energy at    reference energy, which is the output of the gain estimator.

In step 413, the laser is fired. In one embodiment, the laser is firedby controller 130 directing laser source 120 to fire at the laservoltage, that is, the voltage computed by the RRGE corresponding to thetrigger received from scanner 140.

In step 415, the gain estimator is updated. In one embodiment, the gainestimator is updated by controller 130 calculating a new gain estimatorfor the current laser pulse, based on a measured resulting energy fromthe current laser pulse and using gain estimator calculation techniquesknown in the art, and the calculated new gain estimator is then storedin RRGE Table 301 corresponding to the RepRate determined in step 407.In other embodiments all gain estimators are updated on each laser pulseby storing the calculated new gain estimator as new gain estimatorvalues in RRGE Table 301 corresponding to all RepRates in the table.Updating all gain estimators on each laser pulse may provide a moreaccurate gain estimator for each RepRate than using any previouslystored gain estimator (including the default gain estimator).

The process then returns to step 401 to receive a new laser trigger andvoltage command.

In an alternative embodiment, the set of gain estimators is insteadcreated without receiving commands from a scanner. In this embodiment,the voltage command and laser trigger is either generated internallywithin the laser system or by laser system operator input. Regardless ofwhether this embodiment or the one described with reference to FIG. 4 isused, the process of creating and updating the set of gain estimatorscan be considered a training mode for the laser system and, as such,need not actually involve any exposure of the generated laser light to awafer in a scanner.

In an alternative embodiment, a static set of stored gain estimatorscorresponding to various RepRates, previously set and updated asdescribed above with reference to FIG. 4, are then used to fire thelaser without continuing to update the gain estimators as describedabove with reference to step 415 of FIG. 4. In such an embodiment(comprising steps 403 through 413), step 413 of firing the laser thenreturns to step 403 of receiving a laser trigger and voltage commandwithout performing step 415 of updating gain estimators.

The embodiments discussed herein are illustrative of the presentinvention. As these embodiments of the present invention are describedwith reference to illustrations, various modifications or adaptations ofthe methods and or specific structures described may become apparent tothose skilled in the art. All such modifications, adaptations, orvariations that rely upon the teachings of the present invention, andthrough which these teachings have advanced the art, are considered tobe within the spirit and scope of the present invention. Hence, thedescription and the drawing should not be considered in a limitingsense, as it is understood that the present invention is in no waylimited to only the embodiments illustrated.

It is also to be understood that alternative sequences and mathematicalformulas could be used within the spirit and meaning of the presentinvention as described herein.

Likewise, it is to be understood that laser system controller 130 can beany computing system comprising a processor and memory, including apersonal computer, server, or other processing system, that runssoftware instructions for performing the described operations whichinstructions may themselves have come from or reside on a computerreadable storage medium. Alternatively, laser system controller 130 canbe any dedicated hardware such as an application specific integratedcircuit (ASIC) or other hardwired device, with or without firmware, thatis specifically configured to perform the described operations.

What is claimed is:
 1. A method of laser light energy controlcomprising: receiving, in a laser system controller, a first lasertrigger command and a voltage command; converting, by the laser systemcontroller, the voltage command to a first energy target; determining,by the laser system controller, a first laser repetition rate based on adifference between the first laser trigger command and a previous lasertrigger command; retrieving, by the laser system controller, a firstrepetition rate gain estimator corresponding to the first laserrepetition rate; determining, by the laser system controller, a firstlaser voltage using the first energy target and the first repetitionrate gain estimator; directing, by the laser system controller, a lasersource to fire using the first laser voltage; receiving, in the lasersystem controller, a subsequent laser trigger command and a subsequentvoltage command; converting, by the laser system controller, thesubsequent voltage command to a second energy target; determining, bythe laser system controller, a second laser repetition rate based on adifference between the subsequent laser trigger command and the firstlaser trigger command, wherein the second laser repetition rate isdifferent than the first laser repetition rate; retrieving, by the lasersystem controller, a second repetition rate gain estimator correspondingto the second laser repetition rate; determining, by the laser systemcontroller, a second laser voltage using the second energy target andthe second repetition rate gain estimator; and, directing, by the lasersystem controller, the laser source to fire using the second laservoltage.
 2. The method of claim 1 further comprising before the steps ofclaim 1: setting the first repetition rate gain estimator and the secondrepetition rate gain estimator to a default value.
 3. The method ofclaim 2 further comprising: updating the first repetition rate gainestimator after directing the laser source to fire using the first laservoltage and before receiving the subsequent laser trigger command andthe subsequent voltage command; and, updating the second repetition rategain estimator after directing the laser source to fire using the secondlaser voltage and before receiving any other laser trigger command andany other voltage command.
 4. The method of claim 2 further comprising:updating the first repetition rate gain estimator and the secondrepetition rate gain estimator after directing the laser source to fireusing the first laser voltage and before receiving the subsequent lasertrigger command and the subsequent voltage command; and, updating thefirst repetition rate gain estimator and the second repetition rate gainestimator after directing the laser source to fire using the secondlaser voltage and before receiving any other laser trigger command andany other voltage command.
 5. A laser system for laser light energycontrol comprising: a laser system controller configured to: receive afirst laser trigger command and a voltage command; convert the voltagecommand to a first energy target; determine a first laser repetitionrate based on a difference between the first laser trigger command and aprevious laser trigger command; retrieve a first repetition rate gainestimator corresponding to the first laser repetition rate; determine afirst laser voltage using the first energy target and the firstrepetition rate gain estimator; direct a laser source to fire using thefirst laser voltage; receive a subsequent laser trigger command and asubsequent voltage command; convert the subsequent voltage command to asecond energy target; determine a second laser repetition rate based ona difference between the subsequent laser trigger command and the firstlaser trigger command, wherein the second laser repetition rate isdifferent than the first laser repetition rate; retrieve a secondrepetition rate gain estimator corresponding to the second laserrepetition rate; determine a second laser voltage using the secondenergy target and the second repetition rate gain estimator; and, directthe laser source to fire using the second laser voltage.
 6. The lasersystem of claim 5, wherein the laser system controller is furtherconfigured to set the first repetition rate gain estimator and thesecond repetition rate gain estimator to a default value beforereceiving the first laser trigger command and the voltage command. 7.The laser system of claim 6, wherein the laser system controller isfurther configured to: update the first repetition rate gain estimatorafter directing the laser source to fire using the first laser voltageand before receiving the subsequent laser trigger command and thesubsequent voltage command; and, update the second repetition rate gainestimator after directing the laser source to fire using the secondlaser voltage and before receiving any other laser trigger command andany other voltage command.
 8. The laser system of claim 6, wherein thelaser system controller is further configured to: update the firstrepetition rate gain estimator and the second repetition rate gainestimator after directing the laser source to fire using the first laservoltage and before receiving the subsequent laser trigger command andthe subsequent voltage command; and, update the first repetition rategain estimator and the second repetition rate gain estimator afterdirecting the laser source to fire using the second laser voltage andbefore receiving any other laser trigger command and any other voltagecommand.